The present invention relates to a switching power supply apparatus adapted for controlling switching operation of a switching element which carries out switching of rectified and smoothed output of a primary side rectifying/smoothing circuit by a switching control circuit including a hysteresis low voltage malfunction preventing circuit.
Hitherto, there have been widely used switching power supply apparatuses adapted for carrying out switching of direct current obtained by rectifying and smoothing commercial AC (Alternating Current) by, e.g., high frequency of about 100 kHz to efficiently convert it into a desired voltage by transformer.
As a system for controlling output voltage in such switching power supply apparatus, there are employed Pulse Width Modulation (PWM) control system which controls duty ratio of switching pulses in accordance with change of output voltage, and frequency control system or phase control system which controls frequency or phase of switching pulse, etc.
FIG. 1 is an example of circuit configuration of a conventional switching power supply apparatus employing the PWM control system.
This switching power supply apparatus 200 comprises a primary side rectifying/smoothing circuit 215 for rectifying/smoothing AC input delivered through an AC filter 210 from commercial power supply AC, wherein drain of a switching FET 225 is connected to this primary side rectifying/smoothing circuit 215 through a primary winding 220A of a converter transformer 220, and a power supply terminal 230A of a switching control circuit 230 which carries out PWM control of switching operation of the switching FET 225 is connected to the primary side rectifying/smoothing circuit 215 through a starting circuit 240. The power supply terminal 230A is grounded through a capacitor 235.
The switching control circuit 230 includes hysteresis low voltage malfanction preventing circuit therewithin for the purpose of preventing malfunction at the time when power supply voltage is lowered, and is operative so that when power supply voltage Vcc given to the power supply terminal 230A rises from 0V, the operation starts at Vcc=16.5V, and output is cut off (interrupted) at Vcc=9.0V at the time of power supply voltage drop.
A secondary side rectifying/smoothing circuit 250 is connected to a secondary winding 220B of the converter transformer 220 to rectify and smooth converter output obtained at the secondary winding 220B of the converter transformer 220 by the secondary side rectifying/smoothing circuit 250 to output it through an output filter 255. An output detecting circuit 270 is connected to the secondary side rectifying/smoothing circuit 250 through a resistor dividing circuit 260 for output voltage detection and a resistor 265 for output current detection, and detection output by this output detecting circuit 270 is fed back to the switching control circuit 230 through a photo-coupler 280. The output detecting circuit 270 and the photo-coupler 280 become operative with rectified and smoothed output by a rectifying/smoothing circuit 290 connected to the secondary winding 220B of the converter transformer 220 being as drive power supply.
The switching control circuit 230 is started when starting current is delivered through the starting circuit 240 from the primary side rectifying/smoothing circuit 215 at the time of starting to start supply of switching pluses to the switching FET 225, and is operative, after starting, with rectified and smoothed output by a rectifying/smoothing circuit 238 connected to a tertiary winding 220C of the converter transformer 220 being as drive power supply so that duty ratio of switching pulses is changed in accordance with detection output by the output detecting circuit 270 fed back through the photo-coupler 280 to thereby carry out PWM control of switching operation of the switching FET 225 to stabilize converter output.
Meanwhile, in the conventional switching power supply apparatus 200, at the ordinary constant current drooping operation (constant current charging operation of battery), in the case where power of the output detecting control circuit 270 is also taken from the output line, its voltage change width is very wide and a different power supply which can supply stable voltage is required in order to stabilize control. To realize this, devices such as inserting of series regulator, loosening of coupling by different winding of the same transformer to utilize power supply difficult to experience influence of load, or separately providing of rectifying/smoothing circuit even by the same winding, etc. are made to narrow voltage change width even a little to thereby carry out stable control.
Moreover, in the power supply system where power of the output detecting circuit 270 is supplied by different rectification from the same winding of the same transfomer in order to control output of low power switching power supply which carries out intermittent operation in the standby state so that constant voltage and constant current are provided, power necessary for that control is provided by smoothing capacity of the rectifying/smoothing circuit 290 at the time of stop of switching during intermittent operation period. For this reason, capacity of a smoothing capacitor 291 of the rectifying/smoothing circuit 290 becomes large. In addition, since large capacity is required, electrolytic capacitor having good volume capacity ratio is used. For this reason, there also was the problem that change in capacity by change with the passage of time is affected.
In the conventional standby state power saving type switching power supply apparatus, there was employed an approach to detect that there results no-load state to stop switching operation to thereby carry out intermittent operation to carry out power-saving.
As a method of detecting load, there is known a method of inserting resistor in serries with load to detect voltage drop produced across the both ends thereof. In order to detect very small current in the state of light load (about 10 mA) by this method, unless value of detection resistor is set to several ten Ω—several hundred Ω, detection cannot be made with good accuracy. Further, in the case of heavy load, voltage drop and/or exothermic phenomenon at that detection resistor become problem. In the prior art, such problems were solved by a method of shorting detection resistor by semiconductor element. However, the circuit becomes complicated so that there results increase in cost.
In the case where load state is detected to judge that current load state is ordinary load state, light emitting diode of photo-coupler is caused to be turned ON to transmit that signal to switching control circuit of the primary side. Moreover, in the case where it is judged that current load state is no load state, light emitting diode of photo-coupler is caused to be turned OFF to stop switching. In order to carry out such control, it is necessary to carry out transmission by using photo-coupler different from photo-coupler for feedback for constant voltage control. As a result, the circuit additionally becomes necessary.
Further, drive voltage of photo-coupler for judgment of load state becomes short at the time of starting so that its operating state passes through the state where output is not provided. Since such state is judged as no load state, circuit for avoiding it had to be added.
In addition, the photo-coupler is in ON state at all times during ordinary operating state. As a result, extra power is consumed so that there does not result power-saving state at the time of operation.